During the past 10 years, great progress has been made in our knowledge of the planetary atmospheres, mainly due to the recent development of infrared techniques and space astronomy.

Determination of atmospheric temperature structure is of paramount importance to the understanding of planetary atmospheric structure. The most powerful methods for determining atmospheric structure exploit the opacities provided by the collision induced H2 dipole and the ν4 fundamental of CH4. In addition to earth-based observations, useful measurements of thermal emission from Jupiter and Saturn have been or soon will be made by several spacecraft, with results cross-checked with independent radio occultation results. For Uranus and Neptune, only a limited set of whole-disk earth-based data exists. All the outer planets show evidence for stratospheric temperature inversions; temperature minima range from about 105 K for Jupiter and 87 K for Saturn, to roughly 55 K for Uranus and Neptune. In addition to better data, remaining problems may be resolved by better quantitative understanding of gas and aerosol absorption and scattering properties, chemical composition, and non-LTE source functions. Ultimately, temperature structure results must be supplemented by quantitative energy equilibrium models which will allow some meaning to be given to the relationships between such characteristics as temperature, clouds, incident solar and planetary radiation, and chemical composition.

Near infrared spectrophotometry has vastly increased our knowledge of the composition and structure of asteroids, satellites and planetary surfaces over the past ten years. In this article we will attempt to summarize the most recent comprehensive results. We will emphasize the interpretations and present only examples of the data.

Ground-based and spacecraft observations of planets, satellites, and asteroids in the thermal infrared have provided a wealth of information on planetary temperatures, dimensions, and surface properties. Internal heat sources have been revealed for Jupiter, Saturn, and Neptune, and active volcanism on Io has been discovered and monitored. The thermophysical properties of Mars have been mapped for nearly all the surface by spacecraft, and ground-based observations have given similar information for the Galilean satellites of Jupiter. Infrared radiometry thus sheds important light on significant problems of dynamics, interiors, and surfaces of solar system bodies.

The study of infrared sources in molecular clouds necessarily places the practitioner at the interface between two rapidly evolving fields of study. In such a situation, yesterday's heresy often becomes today's dogma. Since it is hard enough to keep up with even one field of study, I thought it might be helpful to recount a bit of history regarding molecular clouds. The objective is to put various notions about molecular clouds into proper context.

The current observational and theoretical literature on Bok globules and their relationship to star formation is reviewed. Recent observations of globules at optical, infrared, and far infrared wavelengths are shown to provide important constraints on their structure and evolutionary status, and the suggestion that many globules are gravitationally unstable is seriously questioned.

Dark clouds associated with T associations are well-known sites of recent and continuing star formation. In recent years molecular observations and far infrared surveys have provided maps of such regions from which possible sites of star formation may be identified. Optical (Hα) and near infrared surveys have enabled a clear identification of pre-main sequence (PMS) objects within the clouds. Methods of distinguishing these from background objects and the nature of their infrared excesses are examined in the light of recent observations in the near and far infrared. The perennial question as to the existence of anomalous reddening within dark clouds is also investigated.

The process of star formation occurs in regions of space not accessible to the traditional techniques of optical spectroscopy and photometry. Star formation begins with a density enhancement in a cold molecular cloud. Stellar gestation then occurs inside an obscuring cocoon of natal gas and dust, which for high mass stars may still be in place when the star reaches the Zero Age Main Sequence (ZAMS). During this time the new star gathers the material which will make up its total mass, solves its angular momentum problem, achieves hydrostatic equilibrium and is making the conversion from gravitational to nuclear energy. The only direct view of this remarkable process is by radio and infrared techniques. Unlike many areas of infrared astronomy, which are extensions of previous optical studies to other wavelengths, information about star formation is primarily gained through infrared and radio studies.

Emission from vibrationally excited molecular hydrogen has been discovered in a variety of objects of widely differing ages and environs including molecular clouds, planetary nebulae, and a Seyfert galaxy. The observations of the H2 spectra indicate this emission arises in hot, nearly thermalized gas. While there is still some disagreement between detailed predictions of hydrodynamic calculations and recent observations, it is generally believed that energy supplied to the interstellar gas in the form of shock waves is responsible for the observed H2 emission.

Several of the H2 sources are molecular clouds associated with ongoing star formation, most notably the Orion Molecular Cloud. From the intensity, strength, temperature, and velocity of the molecular hydrogen emission, it is estimated that at least 1048 ergs has been deposited in the cloud over the last thousand years or so in the form of bulk kinetic energy. There is no clear explanation for this process, since the energy is large and the timescale short, and it appears unlikely that we should observe such events unless they occur frequently. Among the other H2 sources in molecular clouds, NGC 7538, DR 21, and W3 are of similar spatial extent and apparent luminosity as the Orion emission.

The details of the processes which result in the formation of new stars are still virtually entirely obscure. Surprisingly little can be said with any assurance beyond the general view that the interstellar medium collects in large clouds, involving at least tens of solar masses, which then fragment into smaller pieces which proceed to collapse into individual stars. Numerous highly detailed theoretical predictions have been generated for the sequence of events expected in the fragmentation and collapse processes. However, the meager observational evidence we have on suspected young objects is fascinatingly at variance with these predictions.

Recent infrared and radio spectroscopic data pertaining to the Orion BN-KL infrared cluster are reviewed. A new, high resolution CO map shows that the thermal structure over the central 10′(1.5 pc) in the Orion molecular cloud is dominated by energy sources in the infrared cluster and M42. Peak CO brightness temperatures of 90 K occur on KL and near the bar at the southern edge of M42.

Within the central 45″ of the infrared cluster, both radio and IR data reveal a highly energetic environment. Millimeter lines of several molecules (e.g. CO, HCN, and SiO) show emission over a full velocity range of 100 km s−1. These supersonic flows can be modeled as a differentially expanding envelope containing a total of ~5 M⊙ of gas within an outer radius of r ≃ 1.3 × 1017 cm. Over the same area emission is seen from vibrationally excited molecular hydrogen at an excitation temperature of 2000 K. The high velocity mm-line emission and the NIR H2 lines are clearly related since they exhibit similar spatial extents and line widths. Comparison of the total cooling rate for all the H2 lines with the estimated kinetic energy and expansion time for the mm-emission region indicates that the H2 emission probably arises from shock fronts where the expanding envelope impinges on the outer cloud.

Near IR spectroscopy also probes ionized and neutral gas closely associated with BN. Br α and Br γ emission is detected from an ultracompact HII region of mass MHII ≲ 10−4 M⊙. Full widths for the HII lines are ~400 km s−1. CO bandhead emission detected in BN at λ ≃ 2.3 μm is probably collisionally pumped in a high excitation zone (nH+H2 > 1010 cm−3 and TK ≃ 3000 K) at only a few AU from the star. The velocity of both the HII and CO emission is VLSR ≃ + 20 km s−1; thus BN appears to be redshifted by 11 km s−1 with respect to OMC-1.

In recent years there have been a number of extensive review papers and books on interstellar dust (e.g. Savage and Mathis, 1979, and references therein). This short review deals only with the spectral properties of dust within the wavelength range 3–25μm, with particular emphasis on recent results.

Polarization at infrared wavelengths has been detected from a number of different objects within the Galaxy. These include young sources associated with molecular clouds and H II regions, cool stars with thick circumstellar shells, bi-polar nebulae, and normal stars suffering interstellar polarization. Typical levels of polarization detected at 2.2 μm are up to 25% for the molecular cloud sources, less than −5% for the cool stars, around 30% for some bi-polar nebulae and less than −2% for interstellar polarization. For the latter three types of source the origin of the polarization is basically understood: it results from scattering of stellar radiation off small particles in the surrounding shell or nebula in the cool stars and bi-polar nebulae and by transfer of flux through a foreground medium of aligned dust grains for the interstellar polarization. The phenomenon of large infrared polarization in the young stellar and pre-stellar sources is less well understood, and it is to this problem that we address ourselves in this review.

Optical observations provide a wealth of information on atomic abundances, the excitation (level of ionization), the kinetic temperature, and the density of the gas in H II regions. However, obscuration by dust limits optical observations of galactic H II regions to those which are nearby (within a few kpc) and which are unobscured by the molecular clouds out of which they are formed (and so generally evolved). Radio observations are unhampered by extinction but provide much more limited information. The abundance of H+, He+ and occasionally C+ can be determined as well as the kinetic temperature of the gas, but no direct information on the abundance of other species or the excitation is available.

For a long time, the inner region of our Galaxy has been veiled by strong interstellar extinction in visible light. The situation has been greatly improved by recent exploration in the infrared region, where the interstellar extinction becomes practically negligible.

To obtain information on the large-scale structure of our Galaxy, one has to investigate the radiation in the short-wavelength and long-wavelength portion of the electromagnetic spectrum where attenuation throughout the whole galactic disk is very low: gamma-radiation on the one-hand side and infrared and radio radiation on the other side. But since most of the different modes of radiation are generated by interaction of two or more basic galactic constituents one derives only indirect information on a specific component. To come as close to a unique solution as possible the results of as many different spectral regions as possible should be combined. From this point of view it is very encouraging that infrared astronomy has been entering the field and will continue to contribute in the years to come. While gamma-radiation presents mainly information about the cosmic rays and the interstellar gas and radio astronomy about early-type stars and the interstellar gas, infrared astronomy has opened a new way to investigate a major fraction of the stellar population and the interstellar dust. The information is contained in the diffuse galactic emission, which is observed at low galactic latitudes with a field of view large enough to discriminate against point sources, i.e. the emission is averaged over typical dimensions of some 100 pc.

When Gareth Wynn-Williams first asked me to give this summary talk he specifically requested that I be “provocative.” Having already been to Hawaii three times I realized how much easier it is to be laid back rather than provocative here - indeed, I but rarely rise above the ground energy state. So, the prospect of giving this talk has so terrified me that I have been sitting in the front row taking notes for two days now - something I have never done at any meeting. I even sought out the advice of an expert summarizer and first row sitter, my colleague Virginia Trimble, who advised me not to worry, but to simply “tell them what they should have said but didn't.”

Recent infrared and radio observations of the Galactic Center are reviewed. For the region between 1 and 100 pc most of the observed phenomena can be explained by a large density of late-type stars, a ring of molecular material, and a number of regions of active star formation. The central parsec (Sgr A) appears to be a unique region of activity in the Galaxy; this result is based on recent high angular resolution data at 30 to 100 μm and high resolution spectral line observations at 12.8 μm. The observations are discussed in terms of the mass, density structure, and luminosity of the region; the ultimate source of the activity is discussed.

Normal galaxies emit most of their radiation longward of one micron, and many problems related to our understanding of galaxy formation and evolution can be fruitfully addressed with measurements at near-infrared wavelengths. Such problems include the make-up of the red stellar population, the star formation rate, the initial mass function, metallicity effects, and mass-to-light ratio. How these various quantities depend on morphological type, on total mass (or absolute magnitude), on radial position, and on environment is also of great interest. In this review recent infrared observations of extragalactic stars, star clusters, and galaxies having important bearing on these questions are discussed. Particular emphasis is placed on new evidence for the presence of a finite intermediate age population in early-type systems. This evidence comes from observations of intermediate age stars in many Magellanic Cloud globular clusters, observations of such stars in at least one nearby dwarf spheroidal (Fornax), the difficulties of fitting theoretical isochrone models to the red V-K colors of E and SO galaxies, and the differences in the infrared color-magnitude relations for the Virgo and Coma clusters.

“It is not very bright to measure a blue magnitude for a red object.”

– Vera Rubin